From Lisa Grossman on Newscientist.com:
Object: Neptune's tiny moon, Naiad
Size: 100 kilometres across
Favourite game: Hide and seek
Neptune's moon Naiad is a coy sprite. The small moon kept close to its parent planet to hide in the brighter body's reflected glare, and it zipped around at erratic speeds singing "Catch me if you can!" Only once did Naiad stick its head out to wave at a passing spacecraft. It has hidden ever since.
Now after 24 years, the Hubble Space Telescope has found Neptune's long-lost ward – and its strange behaviour hints that the planet's entire family is heading towards chaos.
Naiad was discovered closely orbiting Neptune during a fly-by of the Voyager 2 spacecraft in 1989. Images beamed back to Earth showed the moon to be roughly 100 kilometres wide, with an orbit lasting just 7 hours. Then the probe departed. Despite efforts to find Naiad using ground-based telescopes and a 2004 search with Hubble, no sure sign of the moon turned up.
Mark Showalter at the SETI Institute in California and his colleagues wondered if they could find Naiad using a technique that just recently helped them uncovered an even smaller, entirely new Neptunian moon. They overlaid eight Hubble shots from the 2004 search effort to mimic a much longer exposure and reveal features that were otherwise too faint to see. Sure enough, Naiad showed itself.
Wibbly wobbly
The image-stacking work, presented on 8 October at an astronomy meeting in Colorado, revealed that Naiad had been hidden for so long in part because its orbit is very different from what was predicted based on the Voyager data. Showalter thinks Neptune's other moons are pushing and pulling on Naiad, making its orbit hard to predict.
"There's probably some kind of a perturbation going on that is making its orbit kind of wobble," he says. "Over periods of a decade or so, it speeds up and slows down in a way that is not entirely predictable." These wobbles could be the first indications that Neptune's entire moon system is unstable, he adds, although it could take tens of billions of years for the effects to cause catastrophic collisions.
Now that the image-stacking technique has gained more ground, Showalter plans to turn it on other targets, such as Uranus's missing moon Cordelia, which has only ever been spotted by Voyager 2. "That's my remaining personal challenge," says Showalter. "I'm getting close."